A helicopter rotor hub is the primary structural assembly for driving torque to each rotor blade, balancing centrifugal loads between opposing rotor blades and transferring lift loads to the aircraft fuselage. Common varieties of rotor hubs include articulated, hingeless and bearingless types wherein the rotor hub is characterized by the specific means for accommodating the multi-directional displacement of the rotor blades. For example, articulated rotor hubs typically employ one or more bearing elements to accommodate blade excursions whereas bearingless rotor hubs utilize flexible structures, commonly termed "flexbeams", to functionally replace the bearing elements of articulated rotor hubs.
Within the class or category of articulated rotors are those which include a rotor assembly yoke, one per rotor blade, which is driven by a central hub plate via a spherical multi-laminate elastomeric bearing. More specifically, the hub plate is generally circular in shape and includes a plurality of radially extending spokes and arcuate segments which structurally interconnect adjacent spokes. Furthermore, the spokes and arcuate segments define a plurality of apertures equal to the number of rotor blades. Each of the apertures accepts a rotor hub assembly yoke which is generally C-shaped and circumscribes, in looped fashion, the arcuate segments of the hub plate. Each hub assembly yoke includes a midsection, which extends though the respective aperture, and upper and lower radially extending arms which are disposed on either side of the hub plate and which mount to the respective rotor blade. The spherical elastomeric bearing is interposed between the midsection of each yoke and the respective arcuate segment to accommodate the loads and motions of the associated rotor blade.
Centrifugal forces are transferred to the hub plate as a compressive load in the elastomeric bearing, i.e., as the yoke bears against the outer bearing race of the elastomeric bearing. The spherical configuration of the elastomeric bearing accommodates the transmission of torque to the rotor blade, while furthermore accommodating in-plane (edgewise), out-of-plane (flapwise) and pitch change (feathering) motion of the rotor blade. U.S. Pat. No. 3,761,199, 4,235,570, 4,568,245, 4.797,064, and 4,930,983 illustrate articulated rotors of the type described above and are generally indicative of the current state-of-the art.
The size of the rotor assembly yoke, e.g., the height (spacing between the upper and lower radial arms) and width dimension, is determined by the operational motion envelop, i.e., specific operating requirements, of the rotor system. Accordingly, the flapwise, edgewise and feathering motion of the rotor system determines the clearance requirements between the radial arms and the hub plate. Generally, the motion envelope of, for example, a main rotor assembly prescribes that the height dimension of the rotor assembly yoke be relatively large, i.e., as compared to the height of the hub plate and/or the root end of the rotor blade, to accommodate the large excursions of the rotor blade. For main rotor applications, the height dimension of a typical rotor assembly yoke may exceed 6 inches (15.24 cm) as compared to that of the rotor blade root end which is typically less than 3 inches (7.62 cm).
To accommodate the height differential, various attempts have been made to effect a smooth geometric and structural transition from the rotor blade root end to the upper and lower radial arms of the rotor assembly yoke. FIGS. 1a-1e depict a prior art mounting arrangement for securing a helicopter rotor blade 200 to a rotor hub assembly 202 wherein a torque box member 204 is interposed between the root end 206 of the rotor blade 200 and a rotor assembly yoke 210. A first attachment fitting or pitch horn 212 is interposed between the upper and lower arms 214a, 214b of the rotor assembly yoke 210 and is articulately mounted to a pitch control rod (not shown) for imparting pitch motion to the rotor blade 200. A second attachment fitting or damper lug 216 is formed in combination with the root end 206 of the rotor blade 200, which damper lug 216 engages a damper assembly 218 (see FIG. 1b) for damping i.e., absorbing, the energy associated with self-excited edgewise motion of the rotor blade 200.
In FIGS. 2a-2e, a prior art mounting arrangement of another variety is shown wherein the upper and lower arms 220a, 220b of a rotor assembly yoke 222 are disposed over an integral torque box/damper fitting 224 and, furthermore, are elongated to meet the root end 226 of a rotor blade 228. A damper lug 230 is integrally formed at an outboard end of the torque box/damper fitting 224 while a pitch horn fitting 232 is fastened to a side wall thereof for imparting pitch motion to the rotor blade 228. The torque box/damper fitting 224 is affixed to the yoke arms 220a, 220b at inboard and outboard locations via first and second fastening bolts 236 and 238, respectively and forms an open-end 242 (see FIG. 2a) for accepting the root end of the rotor blade.
FIGS. 3a-3e depict yet another prior art mounting arrangement wherein the root end 250 of a rotor blade 252 is enlarged or expanded to meet the upper and lower radial arms 254a, 254b of a rotor assembly yoke 256. A horn/damper fitting 258 is disposed in combination with the rotor assembly yoke 256 and the rotor blade root end 250 such that the upper and lower transverse straps 260a, 260b of the horn/damper fitting 258 are interposed between the upper and lower radial arms 254a, 254b of the rotor assembly yoke 256 and the rotor blade root end 250 (best shown in FIG. 3d). The transverse straps 260a, 260b converge to form a pitch horn 262 and damper fitting 264 on opposing sides of the rotor blade 252.
The mounting arrangements of the prior art produce large aerodynamic drag penalties due to the high profile area produced by the torque box member 204 (FIG. 1a), the torque box/damper fitting 224 (FIG. 2a) or the enlarged rotor blade root end 250 (FIG. 3a). That is, the geometry and construction of the respective mounting arrangements produces increased profile drag area in regions of high velocity airflow, i.e., as compared to other rotor hub components which lie closer to the rotor system rotational axis 270.
Additionally, the prior art mounting arrangements require precise machining and/or manufacturing tolerances for ensuring proper fit and alignment of the assembled components and for abating the imposition of preinduced stresses in the assembled components. With regard to the latter, the prior art configurations are not forgiving of manufacturing deviations due to the structural rigidity, or stated conversely, the lack of compliance, of the assembled components, e.g., the torque box/damper fitting 224, horn/damper fitting 258, etc. Should manufacturing deviations be present, the mechanical clamping of the assembled components will produce high stress concentrations in areas A (see FIGS. 1e, 2e and 3d) where flexure is induced.
Other disadvantages of the prior art include weight penalties incurred through the use of large structural components, e.g., torque box 204, pitch horn 212, torque box/damper fitting 224 and horn/damper fitting 258, to facilitate mounting of a pitch control rod and/or damper assembly. Furthermore, each mounting arrangement requires the use of filler blocks 272, 274, 276 shown in FIGS. 1e, 2e, 3d, respectively, for supporting the rotor blade root end for mechanically clamping the assembly. Insofar as the filler blocks 272, 274 and 276 are essentially parasitic, i.e., have no utility other than to permit clamp-up of the mounting arrangement, additional weight penalties are incurred.
Moreover, prior art mounting arrangements, particularly those of the type shown in FIGS. 3a-3e, induce high bending moments in the fastening bolts. Referring to FIG. 3e, which shows an enlarged view of the bolted connection 278, it will be apparent by examination of the resultant vectors F that the distance therebetween, which determines the magnitude of the bending moment load, is increased by the thickness of the transverse straps 260a, 260b of the horn/damper fitting. Accordingly, a large bending moment is induced in the bolted connection 278 which necessitates a larger/heavier bolt or, conversely, requires more frequent replacement thereof due to the attendant reduced fatigue life. As a consequence of enlarging the bolt diameter, the overall width dimension of the rotor assembly yoke 256 must increase to maintain the necessary edge distance between the bolt receiving aperture 280 and a free edge 282 of the yoke 256. It will also be appreciated that as the width dimension increases, e.g., by the bolt pattern spacing and edge distance requirements, the height dimension will necessarily increase to accommodate the same pitch motion requirements.
A need therefore exists for a rotor hub assembly which reduces aerodynamic drag, facilitates manufacturing and assembly and is structurally efficient for reducing rotor assembly weight.